Today, we encountered an issue with a FTP client that is trying to connect out to the internet. The FTP client made the initial connection have connected but whenever it tries to transfer files over, it hangs. Since this FTP program isn’t a full FTP client program but a custom made FTP program, it debugging option is limited. We have to use WireShark to monitor the traffic and see what is being blocked. Wireshark is showing the following error when trying to connect:

425: Can’t open data connection

Searching through the web for this error, we noticed that this could be due to the fact that our firewall is blocking the traffic. And it turns out that it was. Once we have opened the required ports between the two servers, we were able to establish the FTP connection.

For those that do not have the option to modify and edit their network firewall rules, another temporary fix is to change your FTP client settings to use PASSIVE mode instead of ACTIVE mode.

Other related errors

150 Opening ASCII mode data connection for /bin/ls
426 Connection closed; transfer aborted